The present invention relates to a device for attachment together of parts of a household electric appliance, at least one of these parts being a metal assembling part in the form of a sheet or plate and having an attachment element projecting from one of the faces of the metal assembling part, the attachment element being formed from the assembling part.
The problem to be resolved is to achieve a sturdy and economical assembly, particularly of an instrument or system, such as a regulator or other part, with an attachment part. The attachment part can be in particular an intermediate part which is in close contact with, and is secured to, by brazing or cementing, a part constituting a wall of a household electric appliance.
No rivet mark may appear on this wall, which must remain fluid tight. Such a wall can notably be that of a tank of an electric boiler or coffee maker, a fryer tank, a steam generator, a clothes pressing iron, a surface cleaner, a water or steam cooker, etc.
The present invention is thus particularly applicable to appliances for heating food where an intermediate part is an aluminum sheet in contact with a stainless steel sheet. The aluminum sheet must distribute the heat dissipated by a heating resistance toward the stainless steel sheet, the latter having a larger surface and being in contact with the food. Stainless steel or another material having comparable properties is selected because it is inert with respect to food with which it is in contact, is resistant to corrosion and scale, is able to withstand mechanical and chemical corrosion, and has an attractive appearance. Use can also be made of a steel sheet coated with a polymer that is compatible with food, for example PTFE, or with any other chemically or electrically applied layer, such as gold, nickel, etc.
Attachment devices that have already been proposed in the art include those in which rivets are obtained by deformation of one of the assembling sheets itself. For this purpose, a projection is formed at the surface of the sheet either by extrusion of a portion of this sheet in a matrix presenting a negative version of the form of the part intended to constitute the rivet, or by semi-stamping of the sheet, or even by localized drawing of the sheet in a die with the aid of a stamp whose diameter is smaller than that of the die.
To attach a part with the aid of such a device, the projecting part of the rivet can be introduced into an opening in the part to be attached, with the axial length of the opening being smaller than the length of this projecting part, followed by deforming, i.e., riveting or crimping, the part of the rivet which extends out of the opening of the part to be attached so as to join the two parts together.
The base of such an integrated rivet is necessarily set back from the level of the support surface of the part in which this rivet has been formed, the support surface of this part corresponding to the face that is opposed to that from which the rivet projects. It is thus not possible to exert on the free surface of this rivet the pressure necessary for crimping, without coming to bear against the withdrawn base of this rivet. As a result, the material of the rivet will be at least partially driven back into the part with which this rivet is integrated; instead of being crimped around the opening of the part to the attached. As a result, it becomes difficult, or even impossible, to obtain an attachment with the desired resistance to pulling apart. This risk of driving back the material of the rivet is that much greater when the material from which the rivet is made is malleable, such as aluminum or particularly copper.
There exist situations in which it is not possible to reach the withdrawn base of the integrated rivet, or in which it is necessary that the support surface of the part in which the integrated rivet is formed not be damaged by the formation of an integrated rivet of the type described above. There also exist cases in which, due to the lack of room above the rivet, it is not possible to utilize a crimping driver which is driven with an oscillating movement, or a driver whose diameter exceeds the diameter of the rivet by more than around 1 mm. Consequently, the head of the rivet can only be produced by a straight line pressing movement and a flat or slightly conical driver. Now, the holding force of the attachment is given by the volume of the rivet head, which is the reason why it is important that the latter not be capable of being pushed back under the effect of the driver pressure, since in this case its volume will be correspondingly reduced to the detriment of the holding strength of the attachment.
There is another factor which must also be taken into account. Given inevitable fabrication tolerances and the fact that, in general, several rivets are necessary to attach two parts together, it is necessary to provide a rivet diameter which is substantially smaller than that of the opening engaged by this rivet. Consequently, during crimping, the volume of the rivet must allow the gaps between the rivet and the opening to be filled and to form a rivet head having a sufficiently large volume. If the cross section of the rivet is smaller than that of the opening engaging the rivet, the rivet volume is thus reduced in such a manner that in order to compensate for this volume reduction, it is necessary to increase the length of the rivet. The difficulty of achieving this increases as the thickness of the base sheet in which the rivet is formed is reduced.
The Patent document FR-258 1571 discloses a process for closing a steam chamber of a steam iron according to which the closing cover of this chamber is pressed on the body of the ironing soleplate with a pressure sufficient to extrude the material of the cover into recesses arranged in the soleplate. Given that the cross section of these recesses is trapezoidal, this cover is permanently fixed in a sealed manner to the ironing soleplate. This attachment is not a riveting and is not practically usable for the type of assemblage that is of interest in connection with the present invention. That publication only discloses an attachment obtained by extrusion of the part to be attached in a recess of the other part to be attached.